The present invention relates to insulated cables and wires for carrying electromagnetic signals.
Although simple, single-strand wire conductors can be used to carry electromagnetic signals, it has long been known that more complex structures or features such as insulators, braided wires, and multiple conductors can be utilized to make connector cables and the like having advanced signal carrying characteristics. This is especially important as the speed and performance requirements of electronic systems continue to dramatically increase. For example, a mere fifteen years ago or so phone service providers only had to worry about a relatively small number of people using 300 baud modems to access remote xe2x80x9cbulletin boardxe2x80x9d services. Now, new computers are capable of transmitting data at 100 or 200 times that rate over standard phone lines (and even more over networks), and the trend to increase speed and performance will likely continue. Cables need to keep pace.
Since multiple signal carrying conductors are often bunched together in one cable bundle, the conductors must also be isolated from one another in order to preserve signal integrity. This is typically accomplished by coating or covering the conductors with some type of insulator. This also protects the conductors from the elements, and, if applicable, reduces the risk of electrical shock. However, because of the nature of electronic signals carried down a conductor (traveling electrons and corresponding electric fields, magnetic fields extending beyond the conductor periphery, etc.), the type and arrangement of insulators surrounding the conductor, as well as the insulators on other, nearby conductors, will have an effect on the signals. One measure of an insulators potential effect on a a conductor""s signal characteristics is given by the insulator""s dielectric constant, which is roughly an indication of how well a material is capable of storing electric charge. Typically, a low dielectric constant will result in lower cable capacitances, and the insulator will xe2x80x9cinteractxe2x80x9d with the signal less, increasing signal propagation speed and increasing signal efficacy. This is especially true if multiple insulated conductors are grouped together because potential capacitances between the conductors will be reduced, and it is also true with respect to co-axial cables, where the electric and magnetic waves propagate down the space between the co-axial cable""s central conductor and outer ground shield.
In order to reduce the dielectric constant of an insulator, it has heretofore been known to use an xe2x80x9cinsulator systemxe2x80x9d to cover the conductor. In a single conductor cable, an insulator system is simply some sort of insulator that is more than just a sheath-like covering in direct contact with the conductor. In a co-axial cable, it is something other than just a solid or foamed-support insulator disposed between the central conductor and outer ground shield. Because two or more materials are typically used in an insulator system, such a system has an effective dielectric constant, which is essentially a composite of the dielectric constants of the different materials taking into account mechanical structure and arrangement. Two examples of such a system for co-axial cables are disclosed in Australian Patent 273087 (xe2x80x9cAU 273087xe2x80x9d) accepted Feb. 13, 1967. The first, which is only briefly mentioned in AU 273087, and which is shown in the instant application as FIG. 1 (labeled xe2x80x9cPrior Artxe2x80x9d), involved wrapping a single plastic or resin strand 24 along the length of the conductor 18 in an open helix or spiral manner. Subsequently, the wrapped conductor was covered by an insulator sheath or tube 20, which contacted the strand 24 instead of the conductor 18 and created an air pocket 22. Thus, the conductor, while still being insulated, was largely surrounded by air, which has a low dielectric constant. However, the effective dielectric constant of this insulator system (insulator strand 24 and sheath 20) was only slightly less than if the sheath was applied directly to the conductor. This is because the relatively large amount of plastic found in the strand (which has a high dielectric constant) partially offset the reduction of the dielectric constant created by the air pocket.
In order to further increase the benefits of the single strand arrangement just described, AU 273087 proposed helically wrapping the conductor with a plurality of stacked and mating rectangular cross section insulators 26, as shown in FIG. 2 of this application (note that the rectangular insulator 26 are not given cross-hatch marks in order to show internal detail). As can be seen, the rectangular insulator 26 were dimensioned to have a height the same as the diameter of the strand 24 in order to sufficiently offset the sheath 20 from the conductor 18. A the same time, the overall amount of plastic found in the air gap 22 was reduced, as shown by comparing the cross-sectional area of the rectangular insulators 26 to the outline 28 of the original strand.
Although the arrangement shown in AU 273087 can be used to reduce the amount of plastic located in the air gap, and therefore the effective dielectric constant of the insulator system, it is disadvantageous for several reasons. First, the stacks of rectangular insulators cannot be used in small cables. For example, as size diminishes, it becomes increasingly more difficult to manufacture and stack the insulators, and to ensure that the insulators are properly wound about the central conductor. With conductors having diameters of 0.015 inch or less, the rectangular stacks of insulators cannot be used without much difficulty and expense, if at all.
Moreover, rectangular insulator stacks are disadvantageous in that the effective dielectric constant of such a system is not adjustable, or only adjustable within a certain range. Specifically, the rectangular insulator stack can theoretically be wound around the central conductor in a tight helix (low pitch), or a loose helix (high pitch). However, in order to ensure structural stability (i.e., proper support for the tubular sheath and outer ground shield, if applicable), the rectangular insulator stack cannot be wound too loosely. Therefore, the effective dielectric constant of the rectangular stack system can be increased (e.g., by winding a tighter helix), but cannot be lowered beyond a minimum value. Although such fine tuning might not have been necessary in the past, for certain applications today (e.g., audio electronics, high speed imaging, computing, radar systems) it is critical.
Accordingly, it is the primary object of this invention to provide an insulator system having a low effective dielectric constant that is still suitable for small cables or conductors.
Another object of the present invention is to further reduce the amount of insulator plastic located between the conductor and the outer insulator sheath, especially in a helically wound insulator system for small cables.
Another object of the present invention is to provide a cable wherein the effective dielectric constant and other characteristics of the insulator system are readily adjustable during manufacturing.
Yet another object of the present invention is to provide a cable wherein the potential for adjusting effective dielectric constant of the insulator system is less constrained by structural stability requirements.
Yet another object of the present invention is to provide a method for adjusting the effective dielectric constant of a signal carrying insulated wire or cable.
Another object of the present invention is to provide a fast co-axial cable having improved structural characteristics and durability.
Another object of the present invention is to provide an improved insulator system, commensurate with the above listed objects, that is easy to manufacture, is inexpensive, and that provides a sturdy covering for a conductor.
An improved coaxial cable for carrying electromagnetic signals, according to the present invention, comprises a central, signal-carrying conductor, and an insulator wrap helically wound around the central conductor. The insulator wrap is formed from a pair of insulator filaments, each having a circular cross section, which are helically entwined or twisted around each other. An insulator sheath or tube surrounds the wrapped central conductor and is supported by the insulator wrap. Since the sheath is offset from the conductor, an enclosed air space is formed between the sheath and the conductor in the space not occupied by the insulator wrap. A concentric, outer ground shield may be disposed about the insulator sheath to provide a co-axial cable, or the insulator sheath alone may serve as an outer cover for the cable.
Because two filaments are used in the insulator wrap, the outer sheath can be offset well away from the central conductor while minimizing the amount of filament material located between the conductor and the insulator sheath. This reduces the effective dielectric constant of the cable, and thereby improves its signal carrying characteristics. Moreover, because the two filaments are twisted around each other, no insulator stacking is required, and therefore the presently disclosed cable can be made as small as desired while still retaining all the benefits of having a reduced effective dielectric constant.
Also, the insulator system cable of the present invention can be provided with a selected effective dielectric constant (some particular value or a minimum value) during the manufacturing process, even if the cable must meet minimum structural stability standards or requirements. Specifically, both the pitch of the entwined filaments of the wrap itself (the xe2x80x9ctwist pitchxe2x80x9d), and the pitch of the wrap about the conductor (the xe2x80x9cwrap pitchxe2x80x9d), determine at how many points the insulator sheath is supported by the wrap, and therefore the cable""s structural characteristics. Thus, given a cable""s structural requirements for a particular application, the insulator system of the present invention, when the twist pitch and the wrap pitch are arranged to minimally meet those requirements (i.e., to provide the minimum number of support points necessary for the particular application), will have the lowest possible effective dielectric constant. Furthermore, both the wrap pitch and the twist pitch can be adjusted up or down, as desired, keeping in mind the overall structural requirements, to vary the effective dielectric constant to some non-minimum value. This is because the effective dielectric constant depends in part on the overall amount of insulator material located between the insulator sheath and the conductor, which is a function of the twist pitch and the wrap pitch. Thus, a manufacturer may precisely adjust the characteristics of the cable (signal speed, etc.), which, as mentioned, is very important for today""s electronic devices or applications.
A cable""s electrical characteristics are interdependently determined, in part, by component dimensions and variances thereof. Accordingly, to maximize performance for high-speed, high-bandwidth applications, and to produce coaxial cables having characteristic impedances with a very low tolerance of xc2x11 ohm: (i) the components in the coaxial cable of the present invention are provided according to very close manufacturing tolerances; and (ii) it has been found that the following component dimensions provide a very-fast (propagation speed of 1.14 ns/ft nominal xc2x10.01 ns/ft for a 50 ohm cable, bandwidth up to 20 GHz), very-small 50 ohm coaxial cable suitable for high bandwidth applications: central conductor=32 AWG-22 AWG; filament diameter=0.0025xe2x80x3-0.010xe2x80x3 (xc2x10.00025xe2x80x3 tolerance); insulator sheath inner diameter=0.020xe2x80x3 to 0.075xe2x80x3 (xc2x10.0005xe2x80x3 tolerance); insulator sheath wall thickness=0.005xe2x80x3 (xc2x10.00025xe2x80x3 tolerance).